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  Method of forming vias in silicon carbide and resulting devices and circuitsUSPTO Application #: 20060065910Title: Method of forming vias in silicon carbide and resulting devices and circuits Abstract: A method of fabricating an integrated circuit on a silicon carbide substrate is disclosed that eliminates wire bonding that can otherwise cause undesired inductance. The method includes fabricating a semiconductor device in epitaxial layers on a surface of a silicon carbide substrate and with at least one metal contact for the device on the uppermost surface of the epitaxial layer. The opposite surface of the substrate is then ground and polished until it is substantially transparent. The method then includes masking the polished surface of the silicon carbide substrate to define a predetermined location for at least one via that is opposite the device metal contact on the uppermost surface of the epitaxial layer and etching the desired via in steps. The first etching step etches through the silicon carbide substrate at the desired masked location until the etch reaches the epitaxial layer. The second etching step etches through the epitaxial layer to the device contacts. Finally, metallizing the via provides an electrical path from the first surface of the substrate to the metal contact and to the device on the second surface of the substrate. (end of abstract) Agent: Summa, Allan & Additon, P.A. - Charlotte, NC, US Inventors: Zoltan Ring, Scott Sheppard, Helmut Hagleitner USPTO Applicaton #: 20060065910 - Class: 257192000 (USPTO) Related Patent Categories: Active Solid-state Devices (e.g., Transistors, Solid-state Diodes), Heterojunction Device, Field Effect Transistor The Patent Description & Claims data below is from USPTO Patent Application 20060065910. Brief Patent Description - Full Patent Description - Patent Application Claims
BACKGROUND [0002] This application is a continuation-in-part of Ser. No. 10/007,431 filed Nov. 8, 2001, which is a continuation of Ser. No. 09/546,821 filed Apr. 11, 2000, now U.S. Pat. No. 6,475,889. [0003] The present invention relates to integrated circuits formed in semiconductor materials and in particular relates to methods for forming via openings in semiconductor substrates, Group III nitride epitaxial layers, and the resulting structures. More particularly, the invention relates to the use of such vias to form monolithic microwave integrated circuits (MMICs) in silicon carbide (SiC). [0004] The present invention relates to the manufacture of via openings ("vias") in integrated circuits (ICs), and in particular relates to a method of forming such vias in devices on a silicon carbide substrate in order to take advantage of silicon carbide's electronic, thermal, and mechanical properties in the manufacture and use of monolithic microwave integrated circuits. [0005] MMICs [0006] In its most basic sense, a monolithic microwave integrated circuit is an integrated circuit; i.e., a circuit formed of a plurality of devices; in which all of the circuit components are manufactured on top of a single semiconductor substrate, and which is designed to operate at microwave frequencies. As is generally the case with integrated circuits, the advantage of placing the device and circuit components on a single substrate is one of saving space. Smaller circuit size offers numerous advantages for electronic circuits and the end-use devices that incorporate such circuits. In general, the end-use devices can be smaller while offering a given set of functions, or more circuits and functions can be added to devices of particular sizes, or both advantages can be combined as desired. From an electronic standpoint, integrated circuits help reduce or eliminate problems such as parasitic capacitance loss that can arise when discrete devices are wire-bonded to one another to form circuits. These advantages can help integrated circuits operate at improved bandwidths as compared to circuits that are "wired" together from discrete components. [0007] Wireless communications systems represent one area of recent and rapid growth in integrated circuits and related commercial technology. Such systems are exemplified, although not limited to, cellular radio communication systems. One estimate predicts that the number of wireless subscribers for such phones will continue to grow worldwide and will exceed 450 million users in the immediate future. The growth of such technologies will require that devices are smaller, more powerful and easier to manufacture. These desired advantages apply to base, relay and switching stations as well as to end user devices such as the cellular phones themselves. [0008] As recognized by those of ordinary skill in this art, many wireless devices, and in particular cellular phone systems, operate in the microwave frequencies of the electromagnetic spectrum. Although the term "microwave" is somewhat arbitrary, and the boundaries between various classifications or frequencies are likewise arbitrary, an exemplary choice for the microwave frequencies would include wavelengths of between about 3,000 and 300,000 microns (.mu.), which corresponds to frequencies of between about 1 and 100 gigahertz (GHz). [0009] As further known by those of ordinary skill in this art, these particular frequencies are most conveniently produced or supported by certain semiconductor materials. For example, although discrete (i.e., individual) silicon (Si) based devices can operate at microwave frequencies, silicon-based integrated circuits suffer from lower electron mobility and are generally disfavored for frequencies above about 3-4 GHz. Silicon's inherent conductivity also limits the gain that can be delivered at high frequencies. [0010] Accordingly, devices that operate successfully on a commercial basis in the microwave frequencies are preferably formed of other materials, of which gallium arsenide (GaAs) is presently a material of choice. Gallium arsenide offers certain advantages for microwave circuits and monolithic microwave integrated circuits, including a higher electron mobility than silicon and a greater insulating quality. [0011] Because of the frequency requirements for microwave devices and microwave communications, silicon carbide is a favorable candidate material for such devices and circuits. Silicon carbide offers a number of advantages for all types of electronic devices, and offers particular advantages for microwave frequency devices and monolithic microwave integrated circuits. Silicon carbide has an extremely wide band gap (e.g., 2.996 electron volts (eV) for alpha SiC at 300K as compared to 1.12 eV for Si and 1.42 for GaAs), has a high electron mobility, is physically very hard, and has outstanding thermal stability, particularly as compared to other semiconductor materials. For example, silicon has a melting point of 1415.degree. C. (GaAs is 1238.degree. C.), while silicon carbide typically will not begin to disassociate in significant amounts until temperatures reach at least about 2000.degree. C. As another factor, silicon carbide can be fashioned either as a semiconducting material or a semi-insulating material. Because insulating or semi-insulating substrates are often required for MMICs, this is a particularly advantageous aspect of silicon carbide. [0012] Advances in semiconductor electronics have increased the availability of wide-band gap materials, such as silicon carbide (SiC) and the Group III nitrides (e.g. GaN, AlGaN and InGaN). The potential for producing transistors operating at high frequencies, including the microwave band, has therefore become a commercial reality. Such higher frequency devices are extremely useful in a number of applications, some of the more familiar of which are power amplifiers, wireless transceivers such as cellular telephones, and similar devices. See generally, commonly assigned U.S. Pat. No. 6,507,046. [0013] The wide bandgap characteristics of silicon carbide and the Group III nitrides enable device manufacturers to optimize the performance of semiconductor electronics at frequencies that traditional materials can not withstand. The high frequency capabilities of these wide bandgap materials present opportunities for development of high frequency, high power semiconductor electronic devices on a scale that will meet the needs of a growing industry. [0014] Wide band gap epitaxial layers of significant interest include the Group III nitrides that are capable of withstanding operation at microwave frequencies. Wu and Zhang explain the operation of these wide bandgap epitaxial layers in international patent application WO 01/57929, assigned to Cree Lighting Company, a wholly owned subsidiary of the assignee herein. Of particular importance to Wu and Zhang are high electron mobility field effect transistors, known as HEMTs. HEMTs, as shown in WO 01/57929, comprise an upper epitaxial layer of semiconductor material on an insulating layer. Source, drain and gate contacts are fabricated on the upper epitaxial layer. The HEMT takes advantage of the physical phenomenon that occurs when two chosen materials of different band gaps are placed in contact with one another in an electronic device. The upper epitaxial layer in an HEMT typically has a wider bandgap than the insulating layer underneath it, and a two dimensional electron gas (2DEG) forms at the junction between the upper epitaxial layer and the insulating layer. The 2DEG formed at this junction has a high concentration of electrons which provide an increased device transconductance. The 2DEG serves as the channel of an HEMT. This channel is open and closed depending on the bias of the signal applied to the gate electrode. See WO 01/57929. [0015] HEMTs are useful in applications that require high power output from a high frequency input signal. HEMT devices can generate large amounts of power because they have high breakdown fields, wide bandgaps (3.36 eV for GaN at room temperature), large conduction band offset, and high saturated electron drift velocity. The same size GaN amplifier can produce up to ten times the power of a GaAs amplifier operating at the same frequency. See WO 01/57929. [0016] The 2DEG of a high electron mobility transistor is essentially an electron rich upper portion of the undoped, smaller bandgap material under the wider bandgap epitaxial layer. The 2DEG can contain a very high sheet electron concentration on the order of 10.sup.12 to 10.sup.13 carriers/cm.sup.2. See commonly assigned U.S. Pat. No. 6,316,793. Electrons from the wider-bandgap semiconductor transfer to the 2DEG, allowing a high electron mobility in this region. Id. A major portion of the electrons in the 2DEG is attributed to pseudomorphic strain in the AlGaN; see, e.g., P. M. Asbeck et al., Electronics Letters, Vol. 33, No. 14, pp. 1230-1231 (1997); and E. T. Yu et al., Applied Physics Letters, Vol. 71, No. 19, pp. 2794-2796 (1997). [0017] High power semiconducting devices, such as the above described HEMT, operate in a microwave frequency range and are used for RF communication networks and radar applications. The devices offer the potential to greatly reduce the complexity and thus the cost of cellular phone base station transmitters. Other potential applications for high power microwave semiconductor devices include replacing the relatively costly tubes and transformers in conventional microwave ovens, increasing the lifetime of satellite transmitters, and improving the efficiency of personal communication system base station transmitters. See commonly assigned U.S. Pat. No. 6,316,793. [0018] Accordingly, the need exists for continued improvement in high frequency, high power semiconductor based microwave devices. One significant improvement described in detail herein is the development of a means for fabricating HEMT devices as part of a monolithic microwave integrated circuit (MMIC). [0019] MMICs are fabricated with backside metallic ground planes, to which contacts must be made from various points in the NMIC, for example at transmission line terminations. Traditionally, this has been accomplished by wire bonds. Although wire bonding techniques can be used for other devices that operate at other frequencies, they are disadvantageous at microwave frequencies in silicon carbide devices. In particular, wires tend to cause undesired inductance at the microwave frequencies at which silicon carbide devices are capable of operating. For frequencies above 10 GHz, wire bonding simply must be avoided altogether. Accordingly, such wire bonding is desirably--and sometimes necessarily--avoided in silicon carbide-based MMICs. [0020] The use of conductive vias (i.e., via openings filled or coated with metal) to replace wire bonds is a potential solution to this problem. To date, however, opening vias in silicon carbide has been rather difficult because of its extremely robust physical characteristics, which, as noted above, are generally advantageous for most other purposes. MMICs that incorporate HEMTs and other semiconductor devices require the additional step of opening vias through the Group III nitride epitaxial layers on the silicon carbide substrate without disrupting device integrity. The invention described herein achieves the opening of conductive vias through the silicon carbide substrate and through the Group III nitride epilayers by utilizing etching techniques tailored to the chemical composition of the substrate and the epilayers. [0021] Etching and Etchants [0022] Etching is a process that removes material (e.g., a thin film on a substrate or the substrate itself) by chemical or physical reaction or both. There are two main categories of etching: wet and dry. In wet etching, chemical solutions are used to etch, dry etching uses a plasma. Silicon carbide does not lend itself rapidly to wet etching because of SiC's stability and high bond strength. Consequently, dry etching is most often used to etch silicon carbide. [0023] In dry etching, a plasma discharge is created by transferring energy (typically electromagnetic radiation in the RF or microwave frequencies) into a low-pressure gas. The gas is selected so that its plasma-state etches the substrate material. Various fluorine-containing compounds (e.g., CF.sub.4, SF.sub.6, C.sub.4F.sub.8) are typically used to etch silicon carbide and different plasma reactor systems may also use gas additives such as oxygen (O.sub.2), hydrogen (H.sub.2), or argon (Ar). The plasma contains gas molecules and their dissociated fragments: electrons, ions, and neutral radicals. The neutral radicals play a part in etching by chemically reacting with the material to be removed while the positive ions traveling towards a negatively charged substrate assist the etching by physical bombardment. [0024] Reactive ion etching (RIE) systems typically use one RF generator. The RF power is fed into one electrode (the "chuck," on which the wafers are placed), and a discharge results between this electrode and the grounded electrode. In such systems, the capacitive nature of RF energy coupling limits the density of the plasma, which in turn leads to lower etch rates of silicon carbide. In RIE systems, plasma density and ion energy are coupled and cannot be independently controlled. When RF input power increases, plasma density and ion energy both increase. As a result, RIE systems cannot produce the type of high density and low energy plasma favorable for etching vias in silicon carbide. Continue reading... Full patent description for Method of forming vias in silicon carbide and resulting devices and circuits Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Method of forming vias in silicon carbide and resulting devices and circuits patent application. Patent Applications in related categories: 20080105900 - Fet channel having a strained lattice structure along multiple surfaces - A channel 16 of a FinFET 10 has a channel core 24 and a channel envelope 32, each made from a semiconductor material defining a different lattice structure to exploit strained silicon properties. A gate is coupled to the channel envelope through a gate dielectric. Exemplary materials are Si and ... ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. 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